Minimally invasive posterolateral fusion

ABSTRACT

A percutaneous system in which tubular retractors are utilized as a means of access, visualization, and a controlled orientation for preparation of posterior spinal elements and delivery of graft material for posterior spinal fusion. The tubular retractors are affixed to screw extensions to rotate and articulate relative to multi-axial screws percutaneously inserted into respective pedicles of vertebral bodies. The procedure is done bilaterally, although it could be performed unilaterally in cases where the anatomy or surgeon preference dictates. A number of different deployable graft concepts are disclosed, the primary features being to allow for creation of a graft space volume and a means for delivery of the graft material preferred by the surgeon.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application No. 62/016,743, filed Jun. 25, 2014, the entire contents of which are incorporated by reference herein.

FIELD OF THE INVENTION

The subject invention relates generally to the field of minimally invasive spinal fusion and more particularly to a minimally invasive method for achieving a posterolateral spinal fusion. The subject technique may be used as an augment for a minimally invasive interbody spinal fusion. It may also be helpful as a sole fusion site for a distal level fusion during cases where interbody fusion may not be achievable, for example in long deformity fusions or very challenging spinal anatomy.

BACKGROUND OF THE INVENTION

Current methods of minimally invasive spinal fusion primarily focus on the interbody space and use of an interbody spinal implant with bone grafting inside the intervertebral disc space. Traditionally, surgeons have performed an open procedure for spinal fusion that involves both the interbody fusion and a posterior lateral fusion. The posterior lateral fusion had been performed an open manner with tissue retraction and direct visualization of the posterior elements including the midline elements of the facet and pars intra-articularis, as well as the more lateral elements of the intertransverse process. With the advent of minimally invasive spinal fusion, direct access and visualization of the midline and lateral elements of the vertebral bodies have been difficult to perform in a minimally invasive manner, while access to the disc has been done with tubular retractors dilating tissues and muscle layers.

A number of investigators have proposed using tubular access to the posterior elements for percutaneous delivery of graft material. However the anchor points for these tubular elements have generally been of a fixed nature and do not allow for the multiaxial motion of the delivery system needed for broad access and visualization of the midline and lateral bony elements. For example, Oktavec et al. in U.S. Patent Application Publication No. 2012/0253316, entitled “Percutaneous Biologic Delivery System”, filed Mar. 28, 2012, disclose such a fixed anchor point where the pivot points are distal from the fixed pin based anchor point and therefore have limited mobility. In addition, the pivoting element is relatively distant from the anchor element, which will tend to result in a more significant retraction of the surrounding tissues and muscle.

Accordingly, there is a need for an improved minimally invasive spinal fusion procedure that will provide for more efficient and desired preparation of the surrounding bony surfaces prior to delivery of an implant and bone graft materials.

SUMMARY OF THE INVENTION

The system of the subject invention provides for minimally invasive access to the posterior spinal elements necessary for bony preparation and graft delivery during posterolateral spinal fusion. A series of tubular retractors is fixed percutaneously to multiaxial screw extensions to provide an access pathway down to the bony elements of the posterior spine. Visualization is achieved through the tubular retractors using either direct vision or endoscopy or microscope magnification. In addition, slots provided along the screw extensions and windows in the retractors provide for additional access points separate from the point of visualization through the tubular retractor. By anchoring the tubular retractors on the multiaxial spinal screw, the surgeon is provided with a well-known access and anchorage point (the spinal pedicle) and the ability to pivot and rotate the tubular retractors as necessary for visualization, bony element preparation, and graft delivery for spinal fusion. An injectable bone graft substitute material such as the geneX® synthetic bone graft (Biocomposites Ltd, Keele, UK) may be delivered directly at the graft site or along the elements spanning two screw-based access points. The tubular retractor has been designed for balancing visualization with minimizing necessary tissue dilation or retraction, as well as for providing a controlled and known access corridor to the posterior spinal elements. Methods for preparing a graft channel between the spinal elements along the midline processes or lateral transverse processes are also provided.

DESCRIPTION OF THE FIGURES

FIG. 1 is a top perspective view of a portion of a spine illustrating partially assembled components a minimally invasive spinal fusion system including a first embodiment of a tubular retractor of the invention.

FIG. 2 is the view of FIG. 1 with the components of the minimally invasive spinal fusion system assembled and attached to the spine.

FIG. 3 is a perspective view of a second embodiment of a tubular retractor.

FIG. 4 is a side elevation view of the tubular retractor of FIG. 3.

FIG. 5 is top perspective view of a third embodiment of the tubular retractor attached to the spine.

FIG. 6 is top perspective view of the tubular retractor of FIG. 5 in use with a tissue preparation instrument.

FIG. 7 is an enlarged top view of FIG. 6 illustrating the visualization of the access port of the tubular retractor as would be seen by a surgeon.

FIG. 8 is a perspective view of a third embodiment of a tubular retractor.

FIG. 9 is top perspective view of the tubular retractor of FIG. 8 in use with a blunt tissue dilator introducer.

FIG. 10 is an enlarged top view of FIG. 9 illustrating the blunt dilator extending through an access port of the tubular retractor as visualized by a surgeon through the access port.

FIG. 11 is a perspective view of a fourth embodiment of a tubular retractor.

FIG. 12 is a top perspective view of a first tissue dilator being inserted through tissue of a patient adjacent to a pedicle screw extension assembly attached to the spine.

FIG. 13 is a top perspective view of a second tissue dilator being inserted through tissue of a patient adjacent to the pedicle screw extension assembly attached to the spine.

FIG. 14 is view of tubular retractor of the second embodiment shown in FIG. 3 slidingly attached to the second tissue dilator in FIG. 13.

FIG. 15 is a side exploded elevation view of the pedicle screw extension assembly, tissue dilator and retractor as shown in FIG. 13.

FIG. 16 is a further top perspective view of FIG. 13 showing a second pedicle screw extension assembly extending through tissue of the patient.

FIG. 17 is a further top perspective view of FIG. 16 with the tissue dilator removed and a second tubular retractor placed over the second pedicle screw extension assembly.

FIG. 18 is an enlarged top view of FIG. 17 is visualized by a surgeon to the access ports of the first and second tubular retractors.

FIG. 19 is a top perspective view of FIG. 17 showing use of an electrocautery device placed directly through a side window of one tubular retractor.

FIG. 20 is a top perspective view of FIG. 17 showing a powered drill placed directly through the pathway of the access port of one tubular retractor.

FIG. 21 is a top perspective view of FIG. 17 showing use of a power drill placed directly through a side window of one tubular retractor.

FIG. 22 is a top perspective view of FIG. 17 showing a blunt dilator attached to and introducer with the blunt dilator being placed through side opening window of one tubular retractor.

FIG. 23 is an enlarged top perspective view of FIG. 17 showing placement of a hollow perforated graft between spinal elements.

FIG. 24 is a view similar to the view of FIG. 23 showing an alternative perforated tube placed between spinal elements for graft material delivery.

FIG. 25 is a view similar to the view of FIG. 23 showing an alternative manufactured perforated anatomical implant followed by injection of flowable biomaterial.

FIG. 26 a is a view similar to the view of FIG. 23 showing an alternative flexible and perforated tubular implant being pushed along a curved rigid access tube.

FIG. 27 is a view similar to the view of FIG. 26 with tubular retractors removed for clarity and with the curved rigid tube in partial cross-section to reveal the tubular implant in the process of being delivered to the spine.

FIG. 28 is a further view of FIG. 27 with tubular retractors in place showing additional bone graft material being injected around the tubular implant.

FIG. 29 is a further view of FIG. 28 showing some of the injected graft material surrounding the previously placed tubular implant.

FIG. 30 is a perspective view of a self-contained delivery device for holding and delivering the tubular implant of FIG. 26.

FIG. 31 is perspective view of another device for more controlled delivery of the tubular implant of FIG. 26.

FIG. 32 is a further view of the delivery device of FIG. 31 showing the distal end of an outer sheath of the delivery device extending through a side window of a tubular retractor.

FIGS. 33 and 34 show the process of withdrawing the outer sheath of the delivery device of FIG. 32 and leaving the tubular implant behind on the spinal elements.

FIG. 35 is an enlarged perspective view of the tubular implant of FIG. 34 after being left behind on the spinal elements.

FIG. 36 is a top perspective view of a pair of bilateral tubular implants and connecting rods having been percutaneously placed on the spine.

FIG. 37 is a top perspective view of a threaded screw tap as an alternative access point to the spinal elements prior to placement of a multi-axial screw.

FIG. 38 is a top perspective view similar to FIG. 2, showing a modified retractor rotatably attached to the tap of FIG. 37.

FIG. 39 shows a top view of the tap and tubular retractor of FIG. 38 depicting the access port as would be visualized by a surgeon.

DESCRIPTION OF THE EMBODIMENTS

For the purposes of promoting and understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains.

As set forth herein, the subject invention provides the ability to visualize and access surrounding bony elements and soft tissues in a mobile manner while utilizing a well-established anchor point in the pedicle of spinal vertebrae. Instrumentation necessary for soft tissue preparation and bony removal may be placed either directly through the access port, through access slots in the side of the tubular retractor, or through slotted elements in the screw extension. This instrumentation provides for efficient and desired preparation of the surrounding tissues prior to delivery of an implant and bone graft materials. The system disclosed hereinbelow for establishing a pathway between adjacent vertebrae comprise a series of blunt dissectors or dilators that may then be followed by a deployable graft implant.

One aspect of the system described herein is in allowing a surgeon to perform a posterior lateral fusion using minimally invasive access and visualization techniques such as a fixed and mobile tubular retractor. In general, the instrumentation and method allows for safe access through the overlying tissue and muscle, direct visualization of the bony elements (e.g. the preparation of the fusion bed), and delivery of bone graft material between multiple spinal segments necessary to achieve spinal fusion. As is well known, a spinal segment comprises opposing vertebral bodies of a spine and the intervertebral disc therebetween.

Turning now to FIGS. 1-2, details of one particular arrangement of a minimally invasive spinal fusion system 1 comprising a pedicle screw extension assembly 10 and a tubular retractor 100 for minimally invasive spine fusion are described. Pedicle screw extension assembly 10 comprises a spinal screw 12 and an elongate screw extension 14 releasbly attached to screw 12. Spinal screw is an elongate fixation member, preferably a multi-axial pedicle screw attached as shown to a pedicle 16 of a vertebral body 18 of a spine 20. Elongate extension 14 defines a longitudinal axis 22 extending generally centrally along the length of extension 14. Extension 14 is generally cylindrical along its outer length and has a pair of flat diametrically opposed surfaces 24 extending lengthwise. A pair of substantially opposite slots 26 extends through flat surfaces 24, each slot 26 communicating with a lumen 28 extending generally centrally through extension 14 along 20. Pedicle screw 12 includes a threaded shaft 12 a and a yoke 12 b articulatingly and rotatably attached to said screw shaft 12 a (see FIG. 15). Elongate screw extension 14 is releasably coupled to yoke 12 b for articulation and rotation of extension 14 about screw shaft 12 a. Details of pedicle screw extension assembly 10 and the arrangement by which elongate extension 14 is releasably coupled to yoke 12 b are described more fully in U.S. Pat. No. 8,845,640, entitled “Pedicle Screw Extension for Use in Percutaneous Spinal Fixation”, issued in the name of Scott Mclean et al. on Sep. 30, 2014 (the '640 patent). The '640 patent is assigned to the same assignee as is the subject application, the contents of the '640 patent being incorporated herein by reference in their entirety.

Referring still to FIGS. 1-2, tubular retractor 100 provides for a closed channel access to the facet region 30 of spine 20, tubular retractor 100 being releasably affixed to the multiaxial pedicle screw 12 through extension 14. Tubular retractor 100 is slid over extension 14 and pushed down through an incision formed through the skin in order to abut against the posterior spinal elements of spine 20. Instruments, as will be described, may be passed through the closed tubular working space in order to access the facet capsule and adjacent midline elements of spine 20.

Tubular retractor 100 is preferably elongate having a distal end 100 a, a proximal end 100 b and an axial length therebetween. Tubular retractor 100 includes an access port 102 including a partially cylindrical wall 103 defining a partially cylindrical pathway 104 extending substantially the entire length of tubular retractor 100, pathway 104 being configured for instrument access and visualization by the surgeon. Pathway 104 defines a second longitudinal axis 106 extending along the length of tubular retractor 100 through distal end 100 a and proximal end 100 b. Tubular retractor 100 includes a pair of generally circular attachment rings 108 a and 108 b disposed respectively at the distal end 100 a and proximal end 100 b. Rings 108 a and 108 b are axially spaced from each other and together define an attachment portion for attaching tubular retractor 100 to elongate extension 14, the axial length of the combined rings 108 a and 108 b being different than and less than the length of access port 102. Each of rings 108 a and 108 b has a curved interior surface for relatively close sliding fit over the generally cylindrical outer surface of extension 14. An interior surface 112 of each of rings 108 a and 108 b is formed as a flat surface to provide a cooperative keying arrangement with one of flat surfaces 24 of extension 14, as will be described. Rings 108 a and 108 b together define a third longitudinal axis 110 that is generally parallel to and offset from second longitudinal axis 106. The open axial space 108 c between rings 108 a and 108 b defines a side opening of tubular retractor 100 that is in communication with pathway 104.

FIG. 2 demonstrates the attachment of tubular retractor 100 to screw extension 14, which is placed down through the same incision of the skin of the patient through which pedicle screw extension assembly 10 extends. During attachment, flat surfaces 112 of rings 108 a and 108 b slidingly engage flat surfaces 24 and extension 14 releasably fixing tubular retractor 100 to extension 14 in a manner to prevent relative rotation between tubular retractor 100 and screw extension 14 while allowing relative axial movement therebetween. Thus, joint articulation and rotation of tubular retractor 100 relative to threaded shaft 12 a of pedicle screw 12 is provided. During attachment third longitudinal axis 110 of rings 108 a and 108 b is substantially coaxial with first longitudinal axis 22 of screw extension 14 with second longitudinal axis 106 of pathway 104 being laterally offset from first longitudinal axis 22. As such, pathway 104 is laterally offset from screw extension 14 and rotatable and articulatable with screw extension 14 about spinal screw shaft 12 a. As depicted in FIG. 2, tubular retractor 100 is fully seated with access port 102 defining a closed working channel pathway 104 having a fixed area transverse to second longitudinal axis 106 for safe access to spinal facets and other posterior spinal elements. Instruments can be placed through pathway 104 to remove soft tissues overlying bony elements of the posterior portion of spine 20, followed by decortication of bony surfaces and placement of a suitable bone graft, as will be described. In addition, tubular retractor 100 can be rotated laterally about first longitudinal axis 22 for access to the transverse processes 32 of the spine 20. Upon rotation of tubular retractor 100 about first longitudinal axis 22 of extension 14 an area of bony surface of spine 20 is exposed adjacent pedicle screw 12 that is greater than the fixed area of said pathway 104. Further, pathway 104 may be accessed through side opening 108 c via slots 26 extending through screw extension 14. Tissue preparation instruments or bone graft material devices may be introduced through side opening 108 c, through slots 26 and into pathway 104 while the surgeon has full access to and visualization of the prepared site through pathway 104 to observe the tissue preparation or bone graft material delivery.

Turning now to FIGS. 3-4, a second alternative tubular retractor 200 for use in minimally invasive spinal fusion system 1 is described. Tubular retractor 200 is elongate having a distal end 200 a and a proximal end 200 b and overall length L₁ extending from distal end 200 a to proximal end 200 b as illustrated in FIG. 4. Tubular retractor 200 comprises an access port 202 and an attachment portion 204. Attachment portion 204 extends longitudinally over the length L₁ and access port 202 extends from distal end 200 a for a length L₂ along tubular retractor 200 toward proximal end 200 b. Length L₂ of access port 202 is different than and less than the length L₁ of attachment portion 204. Length L₁ is adapted to the amount of soft tissue and muscle overlying the spinal elements and necessary to safely reach the site of bone fusion procedure. Attachment portion 204 comprises a generally curved open channel 204 a and defines a longitudinal attachment axis 206 similar to third longitudinal axis 110 defined by attachment rings 108 a and 108 b of tubular retractor 100. Attachment portion 204 has a curved interior surface 204 b for relatively close sliding fit over the generally cylindrical outer surface of screw extension 14 as illustrated in FIG. 3. An interior surface 204 c of attachment portion 204 is formed as a flat surface as depicted in FIG. 3 to provide a cooperative keying arrangement with one of flat surfaces 24 of extension 14, as described hereinabove with respect to tubular retractor 100.

Access port 202 includes a partially cylindrical wall 208 defining a substantially enclosed, partially cylindrical pathway 210 extending the length L₂ of access port 202, pathway 210 being configured for instrument access and visualization by the surgeon. Pathway 210 defines a longitudinal access axis 212 similar to second longitudinal axis 106 defined by access port 102 of tubular retractor 100. Longitudinal access axis 212 is generally parallel to and offset from longitudinal attachment axis 206. Access port 202 has a pair of side openings defined by windows 214 extending through two substantially opposing sides of wall 208 in communication with pathway 210, windows 214 being configured for placement of instruments and implants, as well as for potentially enhanced visualization and illumination.

Pathway 210 of access port 202 has a dimension transverse to longitudinal access axis 212, such as diameter D, as shown in FIG. 3, defining pathway 210 as a fixed, substantially enclosed area for safe access to spinal facets and other posterior spinal elements. Instruments can be placed through pathway 210 to remove soft tissues overlying bony elements of the posterior portion of spine 20, followed by decortication of bony surfaces and placement of a suitable bone graft, as will be described.

The tubular retractors 200 range in sizes adaptable to patient anatomy variations. The diameter D and the length L₂ of access port 202 may vary based on patient anatomy. A surgeon may use different lengths or different diameters depending on the curvature of the spine and amount of overlying soft tissues and muscle. The configuration of tubular tractors 200 as shown in FIG. 3 demonstrates a diameter D ranging from 18 mm to 22 mm, with lengths L₂ from 4 cm up to 8 cm. A kit of different sized tubular retractors 200 and various instruments, dilators and graft implants described hereinbelow may be provided for a surgical procedure. The surgeon may select an access port length L₂ to be as short as possible (just exiting the skin line, S in FIG. 16), in order to maximize the available angular trajectory that may be used for visualization and placement of instruments such as rongeurs or drills for bone decortication. In addition, the presence of side access windows 214 provides for an alternate way to place instruments in a trajectory different from the line-of-sight of the surgeon through pathway 210. A tab 216 projects from wall 208 that allows the surgeon to optionally affix tubular retractor 200 to an operating room table (via a flexible arm assembly not shown) for additional stabilization or for hands-free operation through access port 202.

Turning now to FIGS. 5-7, a third alternative tubular retractor 300 for use in minimally invasive spinal fusion system 1 is described. Tubular retractor 300 includes a modification of tubular retractor 200. Reference numerals illustrating elements of tubular retractor 300 that are common with tubular retractor 200 are increased by 100 for ease of description. In tubular retractor 300, an angular side access port 318 having a guide channel 320 extends from attachment portion 304. A side opening defined by an angular side access channel 320 intersects longitudinal attachment axis 306 of attachment portion 304 and communicates through the slots 26 and lumen 28 of extension 14 with pathway 310 of access port 302 to allow for angled access to the pathway 310 across the screw extension 14. This allows for placement of an instrument 322 such as a drill or burr into the working space of pathway 310 without potentially obstructing the direct visualization of the pathway 310 by the surgeon. In addition, depth stops may be added to instruments 322 placed down angular side access channel 320 for greater control of penetration depth while removing soft tissue and bone prior to graft delivery. The top view of spine 20 as illustrated in FIG. 7 demonstrates placement of the instrument tip 322 a across screw extension 14 and into the operative pathway 310 provided by tubular retractor 300. In this embodiment it is also possible to add a depth stop across the extension slot in order to limit the depth of penetration of the powered instrument. Multiple tissue preparation instruments for preparing bony surfaces or soft tissue for may be placed simultaneously through the three different access openings provided by pathway 310, windows 314 and angular access channel 320.

Referring now to FIGS. 8-10, a fourth alternative tubular retractor 400 for use in minimally invasive spinal fusion system 1 is described. Tubular retractor 400 includes a modification of tubular retractor 200. Reference numerals illustrating elements of tubular retractor 400 that are common with tubular retractor 200 are increased by 200 for ease of description. In tubular retractor 400, an angular window guide funnel 418 having a guide channel 420 communicates with one of windows 414. The configuration of tubular retractor 400 may facilitate a controlled placement of a blunt dilator 422 by an introducer instrument 424 as shown in FIG. 9 capable of creating a subcutaneous tissue pathway between two spinal vertebral bodies as shown in FIG. 10 prior to graft insertion. The shape of guide funnel 418 may assist in reducing tissue migration into window 414.

Turning now to FIG. 11, a fifth alternative tubular retractor 500 for use in minimally invasive spinal fusion system 1 is described. Tubular retractor 500 includes a modification of tubular retractor 200. Reference numerals illustrating elements of tubular retractor 500 that are common with tubular retractor 200 are increased by 300 for ease of description. Tubular retractor 500 may be used for placement of a bone graft material between adjacent spinal levels. An opening 514 a is provided at distal end of windows 514. These respective openings 514 a allow for initially placing a bone graft material in a generally 90° orientation (relative to the skin line S) and then rotating the graft material up to about 90° into position between the distal forks 502 a and 502 b on either side of opening 514 a prior to deploying the graft material between the two spine segments.

Having described various alternative tubular retractors, a method of using such tubular retractors in minimally invasive spinal fusion system 1 is now set forth, with reference primarily, but not exclusively, to tubular retractor 200 and FIGS. 12-21. The primary anchor point for the various steps involved in the access, visualization, preparation, and grafting procedure is based on the fixation of multi-axial pedicle screw 12 into the pedicle of a vertebral body. This is a well-known and established anchor point. Once the tubular retractor 200 is affixed to the screw extension 14, pathway 210 is laterally offset from screw extension 14 and rotatable and articulatable with screw extension 14 about spinal screw shaft 12 a. The surgeon is therefore able to rely upon a fixed access point that is stable and well known anatomically to establish access to bony surfaces of spinal elements surrounding pedicle screw 12. In addition, the multi-axial feature of pedicle screw 14 allows for multiple angles of visualization, preparation, and graft placement as the tubular retractor 200 is rotated and pivoted about the screw shaft 12 a.

The initial step of the procedure involves the formation of small incision I₁ through the skin line S and into the tissue of a patient as shown in FIG. 12. The term “small incision” as used herein is meant to mean an incision of about 4 mm to 25 mm in length sufficient for introduction of the initial dilator 34, and preferably in the range of about 5 mm to 10 mm. A pedicle screw extension assembly 10 is placed through a first incision I₁ with pedicle screw 12 threadedly attached to the pedicle 16 of a vertebral body 18 in a manner as described in the '640 patent, incorporated herein by reference. Screw extension 14 has a length that projects outwardly from skin line S. The skin and tissues adjacent to screw extension 14 are dilated using a series of progressively larger smooth dilators, the initial dilator 34 being shown in FIG. 12. The initial dilator 24 is slid axially adjacent to screw extension 14. This is followed by one or more larger dilators to provide for the exposure necessary for placement of outer tubular retractor 200. The largest dilator 36 is shown in FIG. 13.

Once the tissue has been dilated, tubular retractor 200 may be placed as is illustrated in FIG. 14. Tubular retractor 200 is slid downward in close contact with dilator 36 and screw extension 14 and is manipulated through the skin S and a muscle layers down to the posterior elements of the spine 20. The presence of a tab 216 allows for stabilization of the tubular retractor 200 once fully seated, but also provides the surgeon with an easy handle for pushing down and manipulating tubular retractor 200 through the soft tissues and muscle. FIG. 15 shows the sequential arrangement from the pedicle screw 12 fixed to the vertebral body through the screw extension 14 and tubular retractor 200 and dilators 34 and 36.

At completion as shown in FIG. 16, tubular retractor 200 is placed fully down and seated against the posterior elements of spine 20. At this point, dilators 34 and 36 are removed and an access channel is provided directly to the posterior elements through pathway 210. During placement of tubular retractor 200 over dilator 36 and pedicle screw extension 14 flat interior surfaces 204 c of attachment portion 204 engage one of opposing flat surfaces 24 on screw extension 14 thereby keying tubular retractor 200 to screw extension 14 substantially preventing relative rotation therebetween. Because screw extension 14 is attached to multiaxial pedicle screw 12, tubular retractor 200 may be rotated and pivoted about screw shaft 12 a in order to allow direct visualization and access to the midline elements of spine 20, such as the facet and pars intra-articularis in the medial direction. In addition, tubular retractor 200 may be rotated laterally for direct visualization of the inter-transverse processes.

Multiple tubular retractors 200 may be placed in order to provide access to the posterior spinal elements across one or more spinal segments. As shown, for example in FIG. 16, a second small incision I₂ may be made through the skin line S and into the tissue of a patient at an adjacent vertebral level of spine 20. A second pedicle screw extension assembly 10 may be placed through incision I₂ with pedicle screw 12 threadedly attached to the pedicle 16 of the adjacent vertebral body 18 in the same manner as the attachment of the first pedicle screw extension assembly 10. Likewise a second tubular retractor 200 as shown in FIG. 17 may be slid downward in close contact with a dilator 36 and screw extension 14 and manipulated through the skin S and muscle layers down to the posterior elements of the spine 20 as with first tubular retractor 200. Different sized tubular attractors 200 are shown placed in the regions adjacent to the posterior spinal elements. A top view as shown in FIG. 18 demonstrates the visualization of the spinal elements through the tubular retractors 200 as would be seen by the surgeon. There are multiple sizes of tubular retractors 200 that would be available based on the working space required and the anatomy of the patient and thickness of overlying soft tissues of muscle in the region.

Once tubular retractor 200 has been placed and a visualization pathway is provided down to the posterior spinal elements of spine 20, the next step in the procedure is to remove soft tissues overlying the bony structures. FIG. 19 shows use of an electrocautery device 38 for removal of overlying soft tissues and blood vessel cauterization with the tip of electrocautery device placed directly through side window 214. The soft tissues must be removed prior to bony preparation necessary for securing bleeding decorticated bone along the spinal elements required for achieving spinal fusion. While an electrocautery device 38 is shown, other instruments such as rongeurs and curetttes may also be used to efficiently remove overlying soft tissues under direct visualization by the surgeon.

There are multiple means by which the surgeon may choose to visualize the working space though pathway 210 and to provide for access by the instruments for soft tissue removal and bony preparation. For example, an operative microscope may prove useful for visualization and navigation during the surgical procedure. A surgeon may utilize operative loops that provide for magnification and direct illumination by way of a light source that is fixed to the surgeon's forehead. Finally, a separate endoscopic camera system may be used for illumination and visualization, where the small tubular endoscope would be placed down the working pathway 210 and visualized on a separate monitor within the operating suite.

Instruments may be placed by way of a number of access corridors. FIG. 20 shows a powered drill 40 or burr for controlled preparation and removal of the bone. Drill 40 is placed directly through pathway 210. In FIG. 21 drill 40 is placed through one of the side windows 214 while the surgeon directly visualizes its operation through the pathway 210 of tubular retractor 200.

One of the primary advantages of system 1 is the ability to balance control with a wide range of orientation and access corridors for addressing the adjacent bony elements, both medial (facet, pars interarticularis, lamina) and lateral (transverse processes). Having multiple degrees of freedom provided by the multiaxial pedicle screw 12 provides the surgeon with variations in orientation that are desirable to adequately decorticate the surrounding bony elements prior to graft placement for spinal fusion.

Once the adjacent soft tissues have been removed and the posterior bony elements have been decorticated to expose bleeding bone, the next step is for the surgeon to deposit a suitable bone graft on the prepared bony surfaces. In some cases, it may be adequate simply to place bone graft around the decorticated facet joint to fuse the facet joint. Bone graft material may be placed through a side opening, such as one of windows 214, or through pathway 210 of tubular 200. Facet fusion can provide for some enhanced stability alone or in combination with an interbody spinal fusion. However, in many cases the facet joint on one side of the spine will have been removed in order to allow for passage of an interbody implant. Fusion of the contralateral facet joint alone may provide some benefit, but creation of a bridging bony fusion across two spinal segments on both sides of the spine provides for a more robust and conventional posterior lateral spinal fusion.

In the case where such as a bridging fusion is desired, a dilator, such as blunt dilator 422 described hereinabove with reference to FIGS. 9-10 may be affixed to the an introducer, such as introducer instrument 424 to create a subcutaneous passageway through tissue between the two spinal segments. FIG. 22 shows the placement of blunt dilator 422 through side opening window 214 of tubular retractor 200. It should be appreciated, however, that tubular retractors 400 and 500 as described hereinabove may also be used in the placement of blunt dilator 422 for creating the subcutaneous passageway. Blunt dilator 422 may be placed under direct visualization and then pushed through the soft tissue passageway until it extends into window 214 of the second tubular retractor 200.

FIG. 22 demonstrates the use of multiposition introducer instrument 424 that allows for placement of blunt dilator 422 at multiple orientations. Initially, blunt dilator 422 may be placed in a nearly vertical orientation through incision I₁ down along the sides of the screw extension 14 and into window 214 of tubular retractor 200. Once blunt dilator 422 contacts the posterior bony elements of spine 20 it may be rotated into a 90 degree orientation and pushed subcutaneously across the passageway between the two spinal elements. Multiple sizes of blunt dilators 422 may be used in a progressive manner to safely and gently dilate the tissues between adjacent spinal levels.

Following creation of the tissue passageway between spinal segments, there are number of graft material options that could be useful for bridging the spinal elements, such as those shown in FIGS. 23-31. As described further below, these graft material options include a cylindrical and pliable bone or synthetic graft material tube. The cylindrical graft may be solid, hollow or partially open and can be used to contain an injected graft material, for example. Graft implant designs may also include a more rigid cortical bone, a pliable cancellous bone plug, or synthetic graft material shape to mimic the spinal curvature and provided with a channel and outlets for injection of flowable graft material two fully fill the variable anatomy around the graft implant. Further, a series of sleeves that are pliable and perforated could be placed in a manner analogous to placement of a cardiovascular or urologic stent.

FIG. 23 shows placement of a hollow perforated graft 42 that may allow for a bony incorporation and act as a pathway and scaffold for placement of additional fusion-facilitating materials. Alternatively, a perforated tube 44 could be placed as shown in FIG. 24 (without showing retractors 200 for clarity) allowing for injection of a biomaterial capable of securing bridging spinal fusion. This perforated tube 44 may be removed after placement of the fusion material. Once the tissue passageway is bridged with tube 44, a long cannula 46 may be used for injection of a flowable bone graft substitute. Such flowable bone graft material may include a synthetic hardening bone graft substitute that would set and provide for the appropriate rigidity and stabilization to secure spinal fusion.

A manufactured perforated anatomical implant 48 is shown in FIG. 25. Implant 48 may be fabricated from metal, polymer, ceramic, or a natural material such as allograft bone. Implant 48 would help to maintain the tissue passageway previously created, while providing a scaffold onto which and into which injectable biomaterial could be delivered. Following placement of implant 48 across the spinal segment, additional spinal fusion-enabling material 50 may be placed with direct visualization by an injector 52 through pathway 210 of tubular retractor 200. FIG. 25 demonstrates this method of perforated implant delivery followed by flowable biomaterial injection. The flowable biomaterial could be one of many options available to surgeons. It may involve a setting artificial ceramic material, or could involve the use of a slurry of morselized autograft or allograft or bone marrow aspirate containing mesenchymal stem cells, all capable of injection by way of relatively small diameter cannula 54, as shown.

Alternatively, a flexible and perforated tubular implant 56, as shown in FIGS. 26-27 may be used. This implant could be a synthetic biomaterial or an allograft or autograft plug of cancellous bone. Demineralization of the bone would render it more pliable if needed for insertion down the cannula. In this case, implant 56 is pushed along a curved rigid access tube 58 into the space between the spinal levels. Access tube 58 is placed in the space between the spinal elements as visualized through pathways 210. Curved access tube 58 allows for deposition of implant 56 through a trajectory necessary for bridging spinal elements adjacent two spinal screws 12, as depicted in FIG. 27 showing implant 56 in the process of being delivered to the spine. A flexible tamp 60 is used to push flexible implant 56 into place. Flexible implant 56 may be fabricated, for example, of a partially demineralized cancellous (spongy) bone machined into a semicircular shape and perforated. Once implant is expelled from access tube 58 and is in place flowable biomaterial may be injected. FIG. 28 shows additional bone graft material (such as a ceramic hardening cement or bone marrow aspirate) being injected by injector 52 around implant 56 for an enhanced fusion bed as well as better anchorage of implant 56 at its two opposite ends 56 a and 56 b. FIG. 29 shows some of the injected graft material 62 surrounding the previously placed implant 56. Biomaterial 62 may be injected into and around the perforated implant 56 as shown in FIG. 29 to provide for more surface area, more graft volume, and enhanced anchorage of the implant in the region in which the posterolateral spinal fusion is desired.

Turning now to FIGS. 30-35 an alternative technique for placing a graft implant such as flexible implant 56 is described, with particular reference to tubular retractor 500, as described hereinabove with respect to FIG. 11. As described, tubular retractor 500 may be used for placement of implant 56 between adjacent spinal levels. Opening 514 a at the distal end 500 a of tubular retractor 500 allows for initially placing implant 56 in a 90° orientation (to the skin line S) and then rotating it 90° into position between the distal forks 502 a and 502 b and pushing it down to the prepared bony surfaces through openings 514 a in a manner analogous to the minimally invasive insertion of the connecting rod described in the '640 patent.

A self-contained delivery device 64 for holding and delivering implant 56 is shown in FIG. 30. Device 64 allows for preloading implant 56 (not shown) into a delivery device outer sheath 66 through an opening 68 at the distal end 66 a. Once implant 56 has been loaded the implant 56 may be placed in position between two spinal elements and a plunger 70 at the proximal end 66 b is pushed downwardly in order to displace the implant 56 into the previously created tissue passageway between the vertebral elements.

A delivery device 72 for more controlled delivery of implant 56 is shown in FIG. 31. Device 72 like device 64 allows for preloading implant 56 (not shown) into a delivery device outer sheath 74 through an opening 76 at the distal end 74 a. Distal end 74 a is configured and sized to extend into windows 514 and across tubular retractor 500, as shown in FIG. 32. Once implant 56 has been loaded into outer sheath 74, distal end 74 a may be placed through windows 514 for positioning implant 56 between two spinal elements. In this case, a rather large handle 78 is provided for the surgeon for stabilization and orientation of delivery device 72. A screw thread 80 allows for controlled withdrawal of a collar 82 that will draw outer sheath 74 back and expose implant 56 once it has been docked into the desired location between the spinal elements.

Rather than pushing implant 56 out into the space in which bone graft is desired, delivery device 72 allows the surgeon to essentially “park” implant 56 across the elements in the location desired and then gradually withdraw outer sheath 74 relative to handle 78, leaving implant 56 behind. This process is shown in FIGS. 33-34 prior to withdrawal of outer sheath 74. In FIG. 34 outer sheath 74 has been withdrawn completely leaving implant 56 behind. As depicted in FIG. 34, delivery device 74 is constructed to include an inner access tube 84 which remains stationary relative to handle 78 while outer sheath 74 is withdrawn upwardly over tube 84. Implant 56 engages inner tube 84 and is thereby maintained in place during the outer sheath withdrawal process. FIG. 35 shows implant 56 in enlarged view after being pushed down through opening 514 a of window 514 of tubular retractor 500 toward spinal elements. The dimensions of implant 56 are selected by the surgeon according the anatomy of the patient to span the spinal segment. In the lower lumbar spine, for example, the length of implant 56 may be as large as 60 mm, while in the upper thoracic spine the lengths may be in the range of about 20 to 25 mm. Diameter will range from 5 mm to 15 mm with a diameter of about 9 mm being preferred.

It should now be appreciated that a bone graft material such as bone graft implant 56 can be placed percutaneously across a spinal segment by spinal fusion system 1 as described herein in order to fuse two a more vertebral bodies of spine 20. The placement of graft implant 56 on the transverse processes 86 and 88 of respective vertebral bodies 90 and 92 is illustrated in FIG. 36. Using a pair of pedicle screw extension assemblies 10 and a tubular retractor 500 together with various tissue preparation instruments as described herein, graft implant 56 has been placed in the position shown in FIG. 36 through a small incision, such as incision I₁ or I₂, and then moved subcutaneously through a tissue passageway formed by a blunt dilator 422 adjacent to pedicle screws 12 previously inserted into the pedicles of vertebral bodies 90 and 92. In the case depicted in FIG. 36, implant 56 is disposed laterally of pedicle screws 12 on transverse processes 86 and 88. With the instruments of spinal fusion system 1 and related instruments and methods of use as described herein, implant 56 may have been placed alternatively more midline, such as on the elements of the facet or pars intra-articularis.

With graft implant 56 suitably placed in the desired position on the spinal elements and with the tubular retractor 500 removed from screw extension 14 of pedicle screw extension assembly 10, suitable fixation may also be implanted percutaneously adjacent to graft implant 56. Using slots 26 of screw extensions 14 respectively projecting outwardly from the patient through incisions I₁ and I₂ a connecting rod 94 may be installed in a manner as described in the '640 patent. In such procedure, connecting rod 94 is pivotally attached at its proximal end 94 a to a suitable rod introducer (not shown). The distal end 94 b of connecting rod 94 is inserted through incision I₁ or I₂ and through slots 26 of a first pedicle extension 14 and is then moved subcutaneously through tissue and into and through slots 26 of a second pedicle extension 14. Connecting rod 94 is suitably disconnected from the rod introducer, screw extensions 14 are removed from pedicle screws 12 and incisions I₁ and I₂ are suitably sutured to complete this aspect of the surgical procedure. Another graft implant 56 and connecting rod 94 may be installed on the contralateral side of spine 20 to vertebral bodies 90 and 92 in the same manner as the installation of first graft implant 56 and connecting rod 94. It can thus be appreciated that the entire surgical procedure including the implantation of graft material to promote fusion between vertebral bodies 90 and 92 and associated fixation may be performed percutaneously.

While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. For example, while the placement of the graft implant as well as associated fixation can be placed on the spinal elements in a percutaneous method whereby the graft implants and connecting rods are placed through individual separate incisions, it can be appreciated that the spinal fusion system and instruments described herein may also be used minimally invasively. As such, in situations for instance where the vertebral bodies of the affected spinal segment are relatively close the formation of separate incisions associated with each vertebral body may be difficult, if not impossible, to achieve. In this instance, a single incision, which may be longer than the small incision described hereinabove, may be formed for access to both vertebral bodies with two pedicle screw extension assemblies and tubular retractors introduced through the same incision in a minimally invasive procedure. In some situations, a surgeon may find it advantageous to use the system described herein, or certain of the instruments or tubular retractors for an open spinal procedure.

A further variation within the contemplated scope of the invention is shown in FIGS. 37-39. A primary focus of the spinal fusion system 1 described heretofore rests on utilization of multi-axial pedicle screw 12 as a fixed access point which allows for simultaneous rotational variability of screw extension 14 and an attached tubular retractor, such as tubular retractor 100, 200, 300, 400 or 500. Once the multi-axial screw 12 is placed in the pedicle, it is possible that the screw head could obscure the surgeon's access to some areas of the spinal elements, such as the transverse processes in which there may be interest in removing bone and providing for a bony surface for spinal fusion. FIG. 37 shows the use of a rigid tap 98 as an alternative access point, where tap 98 would be placed prior to placement of multi-axial screw 12. Tap 98 is an elongate fixation member including a distal end 98 a and a proximal end 98 b. Distal end 98 a is threaded fixation element for insertion into a pedicle of a vertebral body. An elongate extension 98 c extends proximally from distal end 98 a defining a longitudinal axis 98 d along the length of extension 98 c. Threaded end 98 a and elongate extension 98 c are fixed together for joint rotation.

It is noted that rigid tap 98 would not provide for the multi axial degrees of freedom and rotation provided by multi-axial screw 12. As such, a modified retractor 600 as depicted in FIG. 38 is contemplated. Tubular retractor 600 includes a modification of tubular retractor 200. Reference numerals illustrating elements of tubular retractor 600 that are common with tubular retractor 200 are increased by 400 for ease of description. Unlike tubular retractor 200, however, attachment portion 604 of tubular retractor 600 does not have a flat interior surface and thereby is not keyed to extension 98 c of tap 98 when tubular retractor 600 is releasably attached to extension 98 c.

Upon attachment of attachment portion 604 to extension 98 c the longitudinal attachment axis of attachment portion is substantially coaxial with longitudinal axis 98 d of extension 98 c and pathway 610 is laterally offset elongate extension 98 c. While threaded end 98 a and extension 98 c are rotationally fixed, attachment portion 604 and thereby tubular retractor 600 with pathway 610 may freely rotate about extension 98 c. Rotation of pathway 610 would allow for a broader area of bony decortication that could potentially be impeded by the presence of the head of pedicle screw 12 introduced later in the procedure. In particular, rotating tubular retractor 600 laterally allows for direct visualization of the inter-transverse process and full decortication of those elements prior to bridging bone graft placement. However in this case, the primary exposure and access needed via tap 98 is to provide for decortication of the transverse process 32 of spine 20. FIG. 39 shows an overview of tap 98 and tubular retractor 600 placed over the tap extension 98 c. Access port pathway 610 provides for orientation and access to transverse process 32.

It should therefore be understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention set forth in the claims are desired to be protected. 

What is claimed is:
 1. A system for minimally invasive spinal fusion, comprising: a pedicle screw extension assembly comprising a multi-axial pedicle screw and an elongate screw extension, said multi-axial screw including a threaded shaft having and a yoke articulatingly and rotatably attached to said screw shaft, said elongate screw extension being releasably coupled to said yoke of said screw for articulation and rotation of said extension about said screw shaft, said elongate extension defining a first longitudinal axis along the length of said extension; and a tubular retractor releasably attached to said extension for joint articulation and rotation with said extension about said screw shaft, said tubular retractor including an access port having a wall defining a pathway therethrough configured for access and visualization, said pathway having a second longitudinal axis substantially parallel to and offset from said first longitudinal axis of said screw extension.
 2. The system of claim 1, wherein said tubular retractor comprises an attachment portion offset from said access port, said attachment portion defining a third longitudinal axis, said third longitudinal axis being substantially coaxial with said first longitudinal axis of said screw extension.
 3. The system of claim 2, wherein said attachment portion of said tubular retractor is cooperatively keyed to said pedicle screw extension in a manner to prevent relative rotation therebetween and allow relative axial movement therebetween.
 4. The system of claim 3, wherein said tubular retractor has a distal end and a proximal end, said access port has a length along said second longitudinal axis and said attachment portion has a length along said third longitudinal axis, the length of said access port and said attachment portion being different.
 5. The system of claim 4, wherein said access port extends longitudinally from said distal end to said proximal end, said attachment portion comprising a first attachment element adjacent said distal end and a second attachment element adjacent said proximal end, said first attachment element being longitudinally spaced from said second attachment element.
 6. The system of claim 5, wherein the longitudinal space between said first attachment element and said second attachment element defines a side opening in communication with said pathway.
 7. The system of claim 4, wherein said attachment portion extends longitudinally from said distal end to said proximal end and said access port extends from said distal end for a length along said tubular retractor toward said proximal end, the length of said access port being less than the length of said attachment portion.
 8. The system of claim 7, wherein said attachment portion comprises a generally curved open channel and wherein said wall of said access port substantially encloses said pathway.
 9. The system of claim 8, wherein said access port has a first side opening defined by a first window extending through said wall in communication with said pathway, said first window being configured for receipt of a tissue preparation instrument and delivery of bone graft material.
 10. The system of claim 9, wherein said access port has a second side opening defined by a second window extending through said wall in communication with said pathway, said second window being disposed substantially opposite said first window.
 11. The system of claim 10, wherein said access port further includes an angular window guide channel communicating with said first window.
 12. The system of claim 10, wherein attachment portion includes a side opening defined by an angular side access port having a guide channel extending through and intersecting said third axis.
 13. The system of claim 12, wherein said elongate extension has a lumen extending longitudinally from said distal end to said proximal end and a pair of opposed longitudinal slots extending through said extension in communication with said lumen, said angular side access guide channel communicating through said slots and said lumen of said extension with said pathway of said access port.
 14. A system for minimally invasive spinal fusion, comprising: an elongate fixation member including a distal end and a proximal end, said distal end defining a fixation element for insertion into a pedicle of a vertebral body, said elongate member including an elongate extension extending from said distal end, said elongate extension defining a first longitudinal axis along the length of said extension; and a tubular retractor releasably attached to said extension, said tubular retractor including an access port and an attachment portion, said access port having a wall defining a pathway therethrough configured for access and visualization, said pathway having a second longitudinal axis substantially parallel to and offset from said first longitudinal axis of said elongate extension, said attachment portion defining a third longitudinal axis that is substantially coaxial with said first longitudinal axis of said screw extension, said attachment portion being attached to said elongate extension for rotation of said tubular retractor about said fixation element, said tubular retractor having a side opening in communication with said pathway.
 15. The system of claim 14, wherein said tubular retractor has a distal end and a proximal end, said access port has a length along said second longitudinal axis and said attachment portion has a length along said third longitudinal axis, the length of said access port and said attachment portion being different.
 16. The system of claim 15, wherein said attachment portion comprises a first attachment element adjacent said distal end and a second attachment element adjacent said proximal end, said first attachment element being longitudinally spaced from said second attachment element, and wherein the longitudinal space between said first attachment element and said second attachment element defines said side opening.
 17. The system of claim 15, wherein said attachment portion extends longitudinally from said distal end to said proximal end and said access port extends from said distal end for a length along said tubular retractor toward said proximal end, the length of said access port being less than the length of said attachment portion.
 18. The system of claim 17, wherein said attachment portion comprises a generally curved open channel and wherein said wall of said access port substantially encloses said pathway.
 19. The system of claim 17, wherein said distal end defining said fixation element and said elongate extension are rotationally fixed such that said fixation element and said elongate extension rotate jointly together.
 20. The system of claim 19, wherein said distal end defining said fixation element comprises a threaded screw tap.
 21. The system of claim 17, wherein said elongate extension is rotatable relative to said fixation element, and wherein said attachment portion of said tubular retractor is keyed to said elongate extension for joint rotation therewith.
 22. The system of claim 17, wherein said side opening is defined by a first window extending through said wall of said access port in communication with said pathway, said first window being configured for receipt of a tissue preparation instrument and delivery of bone graft material.
 23. The system of claim 22, wherein said access port has a second side opening defined by a second window extending through said wall in communication with said pathway, said second window being configured for delivery of bone graft material, said second window being disposed substantially opposite said first window.
 24. The system of claim 23, further including a bone graft delivery device, said delivery device including an outer sheath having a distal end for receiving a graft implant and for depositing said graft implant on spinal elements.
 25. The system of claim 24, wherein said outer sheath of said delivery device is curved.
 26. The system of claim 24, wherein said delivery device includes a plunger for pushing said graft implant from said outer sheath to said spinal elements.
 27. The system of claim 24, wherein said delivery device includes a handle attached to said outer sheath at its proximal end, said outer sheath being movable relative to said handle.
 28. The system of claim 27, wherein said delivery device includes an inner tube in engagement with said graft implant, said inner tube remaining stationary relative to said handle as said outer sheath is moved to thereby withdraw said outer sheath from said graft implant.
 29. The system of claim 28, wherein said distal end of said outer sheath is sized and configured to be introduced into said opposite windows and across said access port.
 30. A tubular retractor for spinal fusion, comprising: an access port including a wall defining a pathway therethrough configured for access and visualization, said pathway defining an access axis and being configured for receipt of tissue preparation instruments and delivery of bone graft material; an attachment portion for attachment to an elongate extension affixed to a vertebral body of a spine, said attachment portion defining an attachment axis substantially parallel to and offset from said access axis; and a side opening in communication with said pathway.
 31. The tubular retractor of claim 30, wherein said tubular retractor has a distal end and a proximal end, said access port has a length along said access axis and said attachment portion has a length along said attachment axis, the length of said access port and said attachment portion being different.
 32. The tubular retractor of claim 31, wherein said attachment portion comprises a first attachment element adjacent said distal end and a second attachment element adjacent said proximal end, said first attachment element being longitudinally spaced from said second attachment element, and wherein the longitudinal space between said first attachment element and said second attachment element defines said side opening.
 33. The tubular retractor of claim 31, wherein said attachment portion extends longitudinally from said distal end to said proximal end and said access port extends from said distal end for a length along said tubular retractor toward said proximal end, the length of said access port being less than the length of said attachment portion.
 34. The tubular retractor of claim 33, wherein said attachment portion comprises a generally curved open channel and wherein said wall of said access port substantially encloses said pathway.
 35. The tubular retractor of claim 33, wherein said side opening is defined by a first window extending through said wall in communication with said pathway.
 36. The tubular retractor of claim 35, wherein said access port has a second window defining a second side opening extending through said wall in communication with said pathway, said second window being disposed substantially opposite said first window.
 37. The tubular retractor of claim 36, wherein access port further includes an angular window guide channel communicating with said first window.
 38. The tubular retractor of claim 36, wherein said side opening is defined by an angular side access guide channel extending through said attachment portion and intersecting said attachment axis.
 39. The tubular retractor of claim 36, wherein each of said windows opens at the distal end of said tubular retractor.
 40. The tubular retractor of claim 30, further including a tab projecting from said wall of said access port for connecting said tubular retractor to an operating table.
 41. A method of minimally invasively fusing vertebral bodies of a spine, comprising the steps of: forming an incision through the skin and into tissue of a patient; attaching through said incision an elongate fixation member to a pedicle of a vertebral body of the spine, said elongate fixation member being releasably secured to an elongate extension defining a first longitudinal axis; introducing a tubular retractor over said extension and moving said retractor through said tissue adjacent said vertebral body, said tubular retractor having an attachment portion and an access port, said attachment portion being releasably attached to said extension, said access port having a wall defining a pathway therethrough configured for access and visualization, said pathway having a second longitudinal axis substantially parallel to and offset from said first longitudinal axis, said tubular retractor having a side opening in communication with said pathway; introducing a tissue preparation instrument through said incision and through one of said tubular pathway or said side opening to prepare a bony surface of said vertebral body for receipt of bone graft material; and delivering through said incision and through one of said pathway or said side opening a bone graft material to said prepared bony surface.
 42. The method of claim 41, further including the step of dilating said incision to expand said incision.
 43. The method of claim 42 wherein said dilating step is practiced by introducing into said incision a plurality of successively larger dilators and then introducing said access port of said tubular retractor over the largest dilator.
 44. The method of claim 41, wherein said elongate fixation member is a multi-axial pedicle screw including a threaded screw shaft and a yoke articulatingly and rotatably attached to said screw shaft, said elongate extension being releasably coupled to said yoke of said screw for articulation and rotation of said extension about said screw shaft, said tubular retractor being releasably attached to said extension for joint articulation and rotation with said extension about said screw shaft.
 45. The method of claim 44, wherein said pathway defines a fixed area transverse to said second longitudinal axis, and wherein said tubular retractor is rotated about said first axis of said extension to expose an area about said pedicle screw that is greater than said fixed area.
 46. The method of claim 45, wherein said side opening is defined by a first window extending through said wall in communication with said pathway.
 47. The method of claim 46, wherein said access port has a second side opening defined by a second side window substantially opposite said first window and extending through said wall in communication with said pathway.
 48. The method of claim 47, wherein prior to placing said bone graft material a blunt tissue dilator is placed through said first window and said second window of said access port to create a subcutaneous passageway through said tissue.
 49. The method of claim 48, wherein said bone graft material is placed subcutaneously through said passageway created through said tissue.
 50. The method of claim 46, wherein said extension has a lumen extending longitudinally therethrough and a pair of opposed longitudinal slots extending through said extension in communication with said lumen, wherein said bony surface is prepared by introducing a tissue preparation instrument through said incision and through one of said pathway, said first and second windows or said extension slots.
 51. The method of claim 50, wherein a connecting rod is introduced through said incision adjacent said extension and is placed through said extension slots.
 52. A method of percutaneously fusing vertebral bodies across a spinal segment, comprising the steps of: percutaneously attaching through a first small incision a first spinal screw to a vertebral body of the spine, said first screw being releasably coupled to a first elongate extension extending through said first incision; percutaneously attaching through a second small incision a second spinal screw to a second vertebral body of the spine, said second screw being releasably coupled to a second elongate extension extending through said second incision, said first vertebral body and said second vertebral body being spaced by an intervertebral disc and defining thereby a spinal segment; and through one of said first incision or said second incision subcutaneously delivering a bone graft material across said spinal segment to a bony area of each of said first vertebral body and said second vertebral body respectively adjacent said first screw and said second screw.
 53. The method of claim 52, wherein said first extension and said second extension are used as a guide to deliver said bone graft material.
 54. The method of claim 53, wherein the step of delivering said bone graft material comprises placing a first tubular retractor over said first extension and through said first incision and into tissue adjacent said first vertebral body and placing a second tubular retractor over said second extension and through said second incision into tissue adjacent said second vertebral body.
 55. The method of claim 54, wherein said first tubular retractor has an attachment portion and an access port, said attachment portion being releasably attached to said first extension, said access port having a wall defining a first pathway therethrough configured for access and visualization, said first pathway being laterally offset from said first extension and rotatable about said first spinal screw, and wherein said second tubular retractor has an attachment portion and an access port, said attachment portion being releasably attached to said second extension, said access port having a wall defining a second pathway therethrough configured for access and visualization, said second pathway being laterally offset from said second extension and rotatable about said second spinal screw.
 56. The method of claim 55, wherein said access port of said first tubular retractor has a first pair of substantially opposite windows extending through said wall in communication with said first pathway, and wherein said access port of said second tubular retractor has a second pair of substantially opposite windows extending through said wall in communication with said second pathway.
 57. The method of claim 56, further including the step removing said first tubular retractor and said second tubular retractor and introducing through one of said first incision or said second incision subcutaneously a connecting rod across said spinal segment and connecting said first screw and said second screw.
 58. The method of claim 53, wherein through one of said first incision or said second incision a connecting rod is delivered subcutaneously to connect said first screw and said second screw.
 59. The method of claim 58, wherein said first extension and said second extension are used as a guide to deliver said connecting rod.
 60. A kit of parts for use in minimally invasive spinal fusion, comprising: a plurality of tubular retractors each including an access port formed by a wall defining a pathway therethrough that is configured for access and visualization, each said pathway defining an access axis, and an attachment portion for attachment to an elongate extension affixed to a vertebral body of a spine, said attachment portion defining an attachment axis substantially parallel to and offset from said access axis, the pathway of each access port having a dimension transverse to said access axis and a length along said access axis, there being a first set of tubular retractors of said plurality having a common transverse dimension and different lengths and a second set of tubular retractors of said plurality having a common length and different transverse dimensions.
 61. The kit of parts of claim 60, further including a plurality of elongate dilators, each having a different length.
 62. The kit of parts of claim 61, further including a bone graft delivery device.
 63. The kit of parts of claim 62, wherein said access port of each tubular retractor has a pair of substantially opposite windows extending through each said wall in communication with each said pathway.
 64. The kit of parts of claim 63, further including a plurality of elongate blunt tissue dilators, each having a different length, connectable to said delivery device and configured for introduction into said windows.
 65. The kit of parts of claim 64, further including a plurality of pre-made, elongate bone graft implants, each having a different length, connectable to said delivery device and configured for introduction into said windows.
 66. The kit of parts of claim 60, further including a plurality of instruments for preparing bony surfaces of a vertebral body of a spine selected from the group of bone removal instruments consisting of drills, decortication burrs, electrocautery devices and of size and configuration for introduction into the pathway of each of said access ports or said windows of each of said access ports.
 67. The kit of parts of claim 66, further including a quantity of injectable bone graft material. 